Preparatory Mindset

AKI is a RAPID decline in kidney function over hours to days. The three categories (pre-renal, intra-renal, post-renal) guide diagnosis and treatment. Pre-renal = decreased perfusion (BUN:Cr >20), intra-renal = direct damage (ATN most common), post-renal = obstruction (USG shows hydronephrosis). Urine electrolytes help distinguish pre-renal from ATN.

Core Concepts

• AKI = rapid ↓ in GFR over hours-days → ↑ Cr and BUN.


• Pre-renal: ↓ renal perfusion (dehydration, heart failure, hemorrhage).


• Lab: BUN:Cr >20, FeNa <1%, urine osmolality >500.


• Intra-renal: direct kidney damage.


• ATN most common (ischemic or nephrotoxic).


• Lab: BUN:Cr <15, FeNa >2%, muddy brown casts in urine.


• Post-renal: obstruction (stones, prostate, tumor).


• USG shows hydronephrosis.


• Complications: hyperkalemia (most immediately dangerous), metabolic acidosis, fluid overload, uremia..

High-Yield Points

• BUN:Cr ratio >20:1 → pre-renal; <15:1 → intra-renal


• FeNa <1% → pre-renal; >2% → ATN (intra-renal)


• ATN: most common cause of intra-renal AKI


• Phases: oliguric → diuretic → recovery


• RIFLE/AKIN criteria for staging severity

Topic Summary

AKI is classified as pre-renal (hypoperfusion), intra-renal (direct kidney damage — ATN most common), or post-renal (obstruction). Treatment addresses the underlying cause and prevents complications.